This invention concerns improvements relating to the propulsion of ships, particularly icebreakers and other vessels used in severe ice conditions, whether for transportation or, say, for arctic exploration.
There is ample experience of ice damage to both fixed-pitch and controllable-pitch propellers fitted to ships operating in heavy ice. This is true even of such propellers which are arranged to operate in nozzles with the object of increasing the so-called "bollard pull" of the vessel at the expense, generally, of a reduction in free running speed.
It is desirable for reasons of propulsive efficiency and maximum thrust when icebreaking that a single propeller be fitted on the centerline of the vessel at a maximum depth of immersion consistent with the ship's keel line, but if a single propeller so fitted is badly damaged by ice, the ship may be immobilized in a geographically remote area. Consequently, partly because of this risk, it is common practice to fit more than one propeller to icebreakers.
If it is possible to fit guards, fore and aft of a nozzle propeller, of sufficient scantlings to keep out large masses of ice and to make the nozzle itself of adequate strength, then it can be postulated that a single propeller be used for the purpose with very much less risk of the ship being disabled by ice. Similar protection would also be desirable for each propeller of a multi-propeller installation. It was, however, to be expected that such guards would reduce propulsive efficiency.